Wireless communications between separated electronic apparatus are widely used. For example, a wireless local area network (WLAN) is a flexible subsystem that may be an extension to, or an alternative for, a wired LAN within a building. In a wireless communication system, the propagation characteristics of the channel or medium through which signals traverse are important factors in the communication system. The medium affects or distorts the transmitted signal in different ways, for example by altering the amplitude and/or the phase of the frequency components. RF technique is well developed for wireless communications. For example, the IEEE 802.11 defines a standard to be followed by RF systems. However, a disadvantage in such system is that the transmitted signal may be significantly distorted by the RF medium as a result of the frequency-dependence of the medium in multipath propagation due to the reception of signal paths other than the line-of-sight (LOS) path. In mobile communication systems, multipath propagations may be the primary cause of distortion in received RF signals, especially in circumstances where there are obstructions in the signal path between the transmitter and receiver. Multipath is caused by the simultaneous reception of signals which result from the RF propagation from the transmitter through a medium with different paths. Particularly, in indoor environments a large number of multiple signal reflections occur when using wireless communication systems. These signal reflections result in a smearing of the signal in time, e.g., time dispersion, and self-jamming. This can adversely impact the performance of a straightforward waveform coding system. The varying path lengths cause differences in the respective received signals in relative delays, amplitude, and/or phase, which combine approximately linearly at the receiver. In general, multipath is a frequency dependent phenomenon, that is, its deleterious effects vary with frequency.
A modulation method used to combat multipath propagation is called spread spectrum. In a spread spectrum communication system, the bandwidth occupied by a digital data is expanded or spread in the transmitter by multiplying the data signal by a spreading signal or sequence. The spreading effect is collapsed in the receiver by the process of correlation. Since multipath is frequency dependent, only part of the spread signal is significantly perturbed. RF communication systems employing spread spectrum techniques are well known and widely used. For example, wireless LAN products frequently employ some type of spread spectrum techniques, such as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS), to communicate between mobile stations and network access points (AP). A distinguishing feature of spread spectrum technique is that the modulated output signals occupy a much greater transmission bandwidth than the baseband information bandwidth requires. Prior art receivers employ matched filters to improve the detection of spread spectrum signals. Matched filtering technique uses an optimum receiver filter which has a frequency transfer function, H*(f), matched to the frequency transfer function, H(f), of the channel, where H*(f) is the complex conjugate of H(f). Spread spectrum communications have been used for various applications, such as cellular telephone communications, to provide robustness to jamming, good interference and multipath rejection, and inherently secure communications from eavesdroppers, for example, in U.S. Pat. No. 5,515,396 to Dalekotzin. U.S. Pat. No. 6,115,411 to van Driest further frees the time interval in a spread spectrum encoder for a WLAN to increase the data rate in the communication system.
The spreading sequence is one type of reference signal in RF communications. It is known advantages to simultaneously transmit a reference signal along with the information-bearing signal in a transmitter system. There are various uses for the reference signal in the receiver system, for example, to accomplish baud and carrier synchronization with the transmitter system. The reference signal may also be used in determining the RF propagation impulse response. For example, U.S. Pat. No. 5,748,677 to Kumar provides a method and system for the simultaneous transmission and reception of a varying reference signal together with the composite data-modulated signal which represents the source message bit information to lower the power for the transmission of the reference signal and improvement of the determination of the transmitted reference signal.
On the other hand, WLANs are usually based on a medium access control (MAC) using a listen-before-talk scheme such as the carrier sense multiple access with collision avoidance (CSMA/CA) as described by the IEEE 802.11 standard. The IEEE 802.11 standard for WLANs is a standard for systems that operate in the 2400–2483.5 MHz industrial, scientific and medical (ISM) band. The ISM band is available worldwide and allows unlicensed operation for spread spectrum systems. The IEEE 802.11 focuses on the MAC and physical layer (PHY) protocols for access points based networks and ad-hoc networks. It supports direct sequence spread spectrum (DSSS) with differential encoded BPSK (DBPSK) and QPSK, frequency hopping spread spectrum (FHSS) with Gaussian FSK (GFSK), and infrared with pulse position modulation (PPM). In particular, under IEEE 802.11b a receiver shall implement a channel clear assessment (CCA) procedure in order to indicate the status of the medium as busy or clear. This CCA mechanism can be achieved by measuring the received energy in the medium and/or by measuring a specific signal pattern in the medium. For the scheme of measuring the received energy in the medium, a technique is used to detect the starting edge of the incoming packet and accumulate received signal energy over some window of length L. An example using the received energy monitoring is provided in U.S. Pat. No. 5,987,033 to Boer et al. by a WLAN with enhanced capture provision. For the other scheme of measuring a specific signal pattern in the medium, there are several different varieties of spread spectrum codes could be used, and the most prominent among them are called m-sequence spreading signals or pseudo noise or PN codes. A variant of those is the Gold codes used in global positioning systems (GPS). There are also Kasami codes and Walsh codes used in IS95 systems, which are not strictly spreading codes but are also listed herewith for reference. In IEEE 802.11b, Barker sequence is the specific signal pattern for the CCA mechanism. The Barker codes were originally developed for radar systems, and they are actually a subset of PN codes and are short codes with a length up to 13. The property that makes them popular for radar system is what is called one-shot correlation, not the end of round correlation. This function has side lobes, e.g., correlation coefficients, of 0 and −1, and this is not the case of PN codes in general. FIGS. 1A and 1B show a basic 7-bit Barker sequence or codeword (+++−−+−) and the auto-correlation on a one shot basis, respectively, by which is shown the main peak at the center and a 0 and −1 cross correlations. As shown, in FIG. 1B, the exemplary timing diagram is for an ideal correlator output, and the Barker sequences causes a waveform spike and fixed-level sidelobes. However, the small sidelobes shown in FIG. 1B are for reference only. The size, shape and spacing of sidelobes are not intended to be to scale. The sidelobes are shown to demonstrate that a correlator generally has some small output value close to zero during symbol processing.
In the related arts, U.S. Pat. No. 5,184,135 to Paradise, U.S. Pat. No. 5,327,496 to Russell et al., U.S. Pat. No. 5,073,898 to Endo et al., U.S. Pat. No. 5,471,497 to Zehavi, and U.S. Pat. No. 5,371,760 to Allen describe the transmissions using PN sequences. U.S. Pat. No. 5,412,620 to Cafarella et al. utilizes a series of chips in a spread spectrum signal. U.S. Pat. No. 5,982,807 to Snell discloses a high data rate spread spectrum transceiver with enhanced overall system performance by use of modified Walsh codes. Further, U.S. Pat. No. 5,909,462 to Kamerman et al. provides a system and method for improved RF receivers that can be trained rapidly to the preamble portion of a data packet in order to use a minimum duration preamble and can be rapidly and accurately trained to a transmitted signal under poor channel conditions. U.S. Pat. No. 5,535,239 to Padovani et al., U.S. Pat. No. 5,416,797 to Gilhousen et al., U.S. Pat. No. 5,309,474 to Gilhousen et al., and U.S. Pat. No. 5,103,459 to Gilhousen et al. disclose CDMA spread spectrum cellular telephone communication systems using Walsh spreading codes. U.S. Pat. No. 6,219,356 to Beukema describes a method for multipath resistant waveform coding, which adds a chip extension to an optimally designed waveform set to compensate for an expected time shift in the radio channel during the transmission and demodulation of the transmitted waveform. The chip extension method can be used in BPSK, QPSK, QBPSK, and a modified Quadrature-BPSK encoding scheme.
There is a need to detect the Barker sequence in the channel or medium in a simple way. Further, it is convenient to report the quality of signal and link such that users can accommodate the physical environment timely to enhance the quality of transmission and receiving. However, prior arts never provide such quality services.